Sewage was treated in anaerobic filters filled with rings of bamboo (either whole or cut), and their effectiveness in the reduction of chemical oxygen demand (COD) was monitored along the entire extension of the filter. This efficiency was determined based on the COD reduction in both the brute effluent (COD-T) and that passed through a glass filter (COD-F). The quantity of total suspended solids (TSS) present at various heights in the filter column, measured at approximately 10-cm intervals from the false bottom to the height of the filter outflow (80 cm above) for various hydraulic detention times (HDT). The performance of the system resulted in little variation up to 5 hours of HDT, with the percentage of reduction of COD-T and COD-F being situated in the range of 60-80% and 40-80%, respectively. The first 40 cm of the filter proved to be relatively effective, and no significant differences in performance between whole and cut rings of bamboo were observed. A shock in pH occurring after 562 days of operation provoked an immediate and marked decrease in the performance of the reactors, especially that operating with a HDT of only 2 hours. This latter filter required more time to return to normal operational conditions than did the other reactors operating with longer HDT.